1. Field of the Invention
The present invention relates to a heater control device for controlling turn-on of a heater, an image forming apparatus, and a computer program product.
2. Description of the Related Art
As a fixing heater used for an electrophotographic image forming apparatus, a halogen heater is widely used. The halogen heater among heaters has a characteristic that inrush current easily occurs particularly at a low temperature, and consumed current is continuously large. Therefore, a voltage drop occurs at a commercial power supply in synchronization with a turn-on timing of the heater, which causes a lighting device such as a fluorescent light to flicker.
There is known a technology for controlling a turn-on pattern on a half-wave cycle basis in a period of 10 half-wavelengths (100 ms) close to a frequency caused to flicker so that turn-on control of the heater is prevented from switching at about 10 Hz band with respect to a flickering frequency band (8.8 to 10 Hz) to which human eyes are sensitive, or so that a frequency component becomes as small as possible (see, e.g., Japanese Patent No. 3316170). This deals with problems such as flicker and control of a harmonic current and of a noise terminal voltage.
Moreover, as a purpose of preventing the inrush current, there is proposed a technology for introducing phase control (soft start) that a heater is turned on for only a part of a half-wavelength right before the high-frequency turn-on pattern and the on-time is made gradually longer (see, e.g., Japanese Patent Application Laid-open No. 2004-212510).
However, in the heater control using the turn-on pattern on the half-wave cycle basis in the period of 10 half-wavelengths, a difference occurs in a flicker level caused by a turn-on duty (turn-on ratio), and it is understood that frequency characteristics are worse in low duty (around 10 to 30%) as compared with that in medium duty (around 40 to 60%) and that the flicker level tends to be bad.
Therefore, there is a problem that inrush current caused by a very short turn-off period (a few 10 msec) during the turn-on cannot be prevented and thus improvement of the flicker level is not expected so much in the low duty in which the turn-off period continues for several half waves or more. Moreover, there is a problem that the number of turn-ons is small in the low duty and the frequency component cannot be improved (frequency cannot be made higher). There is also a problem that when the phase control is used during the turn-on, the characteristics of the harmonic current and the noise terminal voltage may be extremely worsened.